1. Field of the Invention
The invention relates to switched capacitor digital-to-analog converters, and, more particularly, to switched capacitor digital-to-analog converters with power-up/down transient suppression for use in audio systems.
2. Description of Related Art
A number of digital sources of audio information are known. These include compact disk players, digital audio tape, digital transmissions and the like. Stereo digital-to-analog converters are also known, which convert the output from such digital sources into analog information for playback. It is common, when dealing with such stereo digital-to-analog converters, that they have a single ended output ranging between a ground or return value and a supply voltage level with a nominal or quiescent value, Vq, when no signal is applied. It is common in single ended output systems to use a D.C. blocking capacitor to provide a ground centered signal for subsequent processing.
Single-ended digital-to-analog converters (DACs) powered from a single supply can suffer from large transient signals appearing at the outputs when initially powered on. Such DACs present an analog output centered on a nominal quiescent operating voltage, Vq. The transient occurs when power is applied to the part, and the analog outputs are required to move from ground to Vq. If this transient occurs rapidly, it can be approximated as a step function, which has energy at all frequencies. On power-up, such a system can suffer an annoying xe2x80x9cPOPxe2x80x9d at the speaker as the DAC initially charges the D.C. blocking capacitor to Vq.
A similar click or pop can occur when the system is powered off. On entering the power-down state, the charge on the D.C. blocking capacitor remains. When power is removed, the residual charge on the D.C. blocking capacitor discharges rapidly across the load resister resulting in a loud pop.
In accordance with the invention, digital-to-analog converters provide power-up pop/click transient suppression utilizing a digital transient generator which operates to replace the pop or click, which would otherwise occur, with a smooth transition.
In accordance with another aspect of the invention, a digital-to-analog converter suppresses a pop or a click which would otherwise occur when the DAC is powered down using a current source and a positive feedback amplifier.
In accordance with another aspect of the invention, a digital-to-analog converter automatically switches from a base rate mode to a high rate mode by detecting the ratio of the master clock to the left/right clock.